Combustion engines such as diesel engines, gasoline engines, and gaseous fuel-powered engines are supplied with a mixture of air and fuel for combustion within the engine that generates a mechanical power output and a flow of exhaust gases. The exhaust gases can include a complex mixture of air pollutants produced as byproducts of the combustion process. For example, an engine can produce NOx, particulate matter, and hydrocarbons. And due to increased attention on the environment, the amount of pollutants emitted to the atmosphere from an engine can be regulated depending on the type of engine, size of engine, and/or class of engine.
One method that has been implemented by engine manufacturers to comply with the regulation of exhaust emissions includes exhaust gas recirculation (EGR). EGR is the recirculation of a portion of the exhaust gas produced by the engine back to the intake of the engine to mix with fresh combustion air. The resulting mixture, when ignited, produces a lower combustion temperature and a corresponding reduced amount of NOx.
A multi-cylinder engine implementing EGR can include some cylinders that are designated as “donor” cylinders, and other cylinders that are designated as “non-donor” cylinders. Donor cylinders donate all or part of their exhaust for the purpose of EGR. In contrast, non-donor cylinders do not donate any exhaust for EGR purposes. Instead, the exhaust from non-donor cylinders is directed to the atmosphere. This type of engine is known as a donor engine, and control over EGR in a donor engine may be simplified when compared to a conventional engine.
Another method of engine control implemented by engine manufacturers is known as “skip firing”. Skip firing includes the selective deactivation of some of the cylinders of an engine during low-load operations. By deactivating some of the cylinders, the remaining active cylinders must produce more power in order to still satisfy a given load. And operating the remaining cylinders at the higher power output may result in a more efficient combustion process that produces lower levels of particulate matter and hydrocarbons. In addition, a fuel savings may be realized during skip firing in some applications.
An exemplary engine implementing both EGR and skip firing is disclosed in U.S. Patent Application 2012/0298070 of Akinyemi et al. that published on Nov. 29, 2012 (“the '070 publication”). In particular, the '070 publication discloses an engine having twelve cylinders divided into two banks (e.g., a first bank having cylinders 1-6, and a second bank having cylinders 7-12), wherein four of the cylinders (e.g., cylinders 2, 5, 9, and 10) are designated as donor cylinders. A normal-load firing order of the engine is 1-7-5-11-3-9-6-12-2-8-4-10. The engine selectively deactivates six of the cylinders at low engine load, so as to raise the load on the remaining six active cylinders. Specifically, during a first engine cycle, only cylinders 1-5-3-6-2-4 are fired, while in the ensuing engine cycle, only cylinders 7-11-9-12-8-10 are fired. Each cylinder is fired once every two engine cycles. Different patterns of skip firing may be implemented to provide desired levels of EGR.
Although the engine of the '070 publication may have improved emissions and fuel economy in some situations, it may also suffer drawbacks in other situations. For example, the engine of the '070 publication may be prone to vibration-related failures (e.g., engine mount failure), high noise, and/or erratic emissions output. Specifically, when using the conventional firing order listed above, the engine may alternate cylinder firing evenly between the first and second cylinder banks. That is, for the firing order 1-7-5-11-3-9-6-12-2-8-4-10, the engine will fire a first bank cylinder, a second bank cylinder, a first bank cylinder, etc. Likewise, every third firing cylinder may be a donor cylinder. Accordingly, the conventional firing order may result in a mechanically balanced engine having low vibration and fairly consistent EGR flow rates. However, when skip firing using only six cylinders to power a load in the firing order listed above, the engine would suddenly be required to fire all first bank cylinders during the first engine cycle and then all second bank cylinders during the second engine cycle, which is mechanically imbalanced and has the potential to induce severe vibrations within the engine. In addition, during the first engine cycle, one donor cylinder will fire, followed by two non-donor cylinders, followed by another donor cylinder. And in the second engine cycle, two non-donor cylinders will fire, followed by two donor cylinders, followed by two non-donor cylinders. And at the transition between cycles, three non-donor cylinders will fire consecutively. This variability in EGR exhaust donation could cause erratic swings in the levels of regulated exhaust constituents being produced and discharged to the atmosphere.
The disclosed engine system is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.